1. Field of the Invention
The invention relates to light sensitive coating compositions useful for the preparation of lithographic printing plates, color proofing films, resists and the like.
2. Description of Related Art
The art of lithographic printing is based upon the immiscibility of oil and water, wherein the oily material or ink is preferentially retained by the image area and the water or fountain solution is preferentially retained by the non-image area. When a suitably prepared surface is moistened with water and ink is applied, the background or non-image area retains the water and repels the ink while the image area accepts the ink and repels the water. The ink on the image area is then transferred to the surface of a material upon which the image is to be reproduced, such as paper, cloth and the like. Commonly the ink is transferred to an intermediate material called the blanket which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
A very widely used type of lithographic printing plate has a light-sensitive coating applied to an aluminum base support. The coating may respond to light by having the portion, which is exposed become soluble so that it is removed in the developing process. Such a plate is referred to as positive-working. Conversely, when that portion of the coating which becomes exposed becomes hardened, the plate is referred to as negative working. In both instances, the image area remaining is ink-receptive or oleophilic and the non-image area or background is water receptive or hydrophilic. The differentiation between image and non-image areas is made in the exposure process where a film is applied to the plate with a vacuum to insure good contact. The plate is then exposed to a light or energy source. The light source may be primarily in the UV, visible or IR portions of the electromagnetic spectrum. In the instance where a positive plate is used, the area on the film that corresponds to the image on the plate is opaque so that no light will strike the plate, whereas the area on the film that corresponds to the non-image area is clear and permits the transmission of light to the coating which then becomes more soluble and is removed. In the case of a negative plate the converse is true. The area on the film corresponding to the image is clear while the non-image area is opaque. The coating under the clear area of the film is hardened by the action of light while the area not struck by light is removed. The light hardened surface of a negative plate is therefore oleophilic and will accept ink while the non-image area which has had the coating removed through the action of a developer is desensitized and is therefore hydrophilic.
Direct digital imaging of offset printing plates has become increasingly important in the printing industry. Advances in solid-state laser technology have made medium to high powered diode lasers attractive energy sources for platesetters, particularly lasers emitting energy in the blue region of the visible spectrum (400-410 nm) although imaging at other wavelengths may advantageously be performed such as 488 nm and 532 nm. The use of controlled laser exposure obviates the need to use a film or mask when making image exposures, thereby facilitating a platemaking operation.
There are a number of United States patents relating to imaging compositions sensitive to various wavelengths in the visible and near UV regions of the electromagnetic spectrum which contain binder resins, ethylenically unsaturated monomers, photoinitiators and are negative working. A negative working plate based upon a single photosensitive layer using an ethylenically unsaturated monomer which is a phosphate compound having at least one (meth)acryloyl group, photopolymerization initiator and a polymeric binder is disclosed in U.S. Pat. No. 6,514,668. U.S. Pat. No. 6,232,038 discloses a photosensitive composition comprising an ethylenically unsaturated bond-containing compound, a sensitizing dye and a photopolymerization initiator which is either a boron complex or halomethyl group containing compound. Disclosed in U.S. Pat. No. 5,922,508 is a dual layer system wherein the first layer is photopolymerizable and comprised of a binder, photoreducible dye, photopolymerizable initiator and an ethylenically unsaturated monomer having at least one unsaturated group and one photooxidizable group. This coating is overcoated with a polymer/dye combination which has a window of transparency between 300 and 700 nm but is opaque between 350 and 400 nm. U.S. Pat. No. 5,912,105 discloses a dual coated product wherein the first layer is photosensitive and contains a polymeric binder, a polymerizable compound with at least two unsaturated groups and a free radical generating initiating system. It is overcoated with an oxygen barrier layer.
One of the problems associated with these and similar systems is that often there is insufficient integrity of the image areas remaining after development of the printing plate to effectively perform the printing process over long printing runs, thereby resulting in print images having less than desired resolution and print quality. Another problem is that at certain wavelengths (i.e. 400-410 nm), the photospeed required of the plate is very high. Lasers in this region have a low effective output thereby requiring that the plate have inherently greater photospeed. It can be difficult to have a plate product able to satisfactorily be imaged at such low power levels.
The object of the present invention is to provide an element capable of providing long runs, while having high resolution and high photosensitivity.